Project Summary/Abstract The collagen fibers that span the periodontal ligament (PDL) connect teeth to the bone socket by weaving through the cementum of each tooth as well as into the alveolar bone. Collagen type I is the main structural component of PDL. High rates of extracellular matrix (ECM) turnover are characteristic of PDL tissue. Periodontal disease (PD) afflicts approximately 50% of the population in the United States. PD is marked by chronic inflammation of the periodontium leading to PDL degradation, alveolar bone loss, and eventual tooth loss. There are currently no accepted methodologies to regenerate this collagenous PDL tissue. Thus PDL provides an excellent tissue milieu for investigating mechanisms of collagen processing and assembly during inflammatory states that are clinically relevant. SPARC, a collagen-binding protein, has been identified as a key factor in collagen ECM deposition. We reported that SPARC-null mice have significantly less total collagen, thinner collagen fibers, and reduced mechanical strength in PDL compared to wild type (WT) PDL. A key factor in incorporation and stabilization of insoluble collagen within the ECM is mediated through collagen crosslinking. Transglutaminases (TGs) are a family of extracellular proteins known to participate in collagen cross-linking activity in vitro and in vivo. Previous data implicate SPARC as a critical regulator of TG activity on collagen I in homeostatic PDL. Inhibition of TG activity in vivo was shown to reverse SPARC-dependent decreases in collagen volume fraction and mechanical strength of homeostatic PDL. Our data predicts SPARC regulates TG activity by mediating substrate specificity in the ECM, however, the mechanisms by which cross- links influence collagen architecture and repair during and following inflammatory injury is unknown. We hypothesize that increases in tissue TG (TG2) activity in response to injury diminishes collagen fiber content and mechanical strength in the PDL and that manipulation of TG2 activity through either expression of SPARC or pharmacological intervention will improve collagen deposition and repair in a model of PD. We will investigate our hypothesis through the following Specific Aims: 1. Using a clinically relevant murine model of PD, demonstrate TG2 activity is increased in WT mice with PD and further enhanced in the absence of SPARC, 2. Demonstrate that collagen assembly and mechanical strength of healing PDL is decreased by increased TG activity, and 3. Demonstrate that loss of SPARC expression in fibroblasts drives increased TG-dependent cross-linking of collagen I in PD. This project will determine the role of SPARC in the regulation of TG2-mediated collagen cross-links during and following inflammatory PDL injury. The training plan proposed here will test the above hypothesis and prepare the applicant for a successful career as an academic scientist.